Structures such as roadways, railways, embankments, and levees must often be built on soil structures which are insufficiently strong to support their intended loads, immediately or after a considerable passage of time. Yet physical constraints require that these projects be built there. For example, a road must skirt a hill, pass through a meadow, or pass through a soggy a plain next to it. The alternative would be an unaffordable tunnel. Or perhaps a levee must be built next to a river to protect a city. The alternative of moving a city such as New Orleans itself so as to locate a levee at a more convenient location is not even to be considered.
Accordingly, means have been sought to strengthen existing soil structure so that when it is modified it will be able vertically to support or laterally to resist design loads that the existing soil could not have supported. One well-known technique is to mix cement or lime (or both) into the soil so as to make a stronger subsurface structure. Perhaps the best known, or at least the most frequently-encountered, example is to add a reinforcing binder such as cement and/or lime into the existing soil in a vertical bore so that when the resulting mixture cures, it forms an in-situ piling. This piling is thereby constructed of the existing soil as an aggregate plus the added binder. Of course, the ultimate strength of the piling can be attained only if water already exists along with the aggregate, or is supplied when the piling is formed. The ultimate objective is to make a cementitious stoichiometric mix of water and binder that will in time harden to its best properties.
In addition, the strength properties of the piling depend strongly on the amount of binder supplied to it. A stoichiometric mixture merely requires sufficient water to cure the amount of binder that is supplied.
An example of in-situ piling is shown and described in applicant's U.S. Pat. No. 5,967,700 issued Oct. 19, 1999. It is the sense of this patent to know the need for water at various elevations in the intended structure, and to mix it in with the existing soil on the way down into the soil, and then on the way up, to mix in the necessary amount of cement or lime to form a piling of desired characteristics.
Such pilings, and also those made with the present invention, should not be confused with conventional pilings that are prepared off-site. Conventional pilings brought to the site are then and there driven into the soil. These are sometimes lengths of timber. Other times they are poured and cured concrete structures, all with very substantial compressive, shear, and fracture strength. They do not integrate themselves in the soil structure into which they are driven, nor do they include any part of the existing soil in themselves. Instead they exist as free-standing foreign bodies. They are costly to manufacture, transport to the site, and drive into the ground. Their cost, and to a surprising extent, their excessive physical properties lead engineers to use them sparingly. For piers, building foundations, and the like, their use is economically justified. However, to provide many of them per mile for many miles of a roadway or levee can rarely be justified. Also, their inherent strength is much greater than needed for purposes of this invention.
The preparation of an in-situ piling is inherently less costly than the use of a piling prepared off-site. It requires only an auger/mixer to drill into the soil structure and inject and mix materials which, along with whatever soil is already present, will hopefully cure to a solid in-situ vertical piling which has stronger properties than the surrounding soil. Furthermore, its boundaries with surrounding soil structure will not be as abrupt as those of a driven piling. Instead, when properly made, the boundary is likely to be a gradual transition. These are fundamental considerations when one decides to provide an in-situ piling, and how to make it.
The genuinely surprising fact exists that in practice in-situ pilings have not been built to their anticipated strength levels, even when these levels were known, which is not necessarily a uniform property throughout the depth. In fact this shortcoming has not been widely noticed, nor in most practice has it been recognized as a problem. Generally the concept has been to put a calculated amount of a binder in the bore, mix it into the soil, and leave.
The necessary properties of an in-situ piling are surprisingly less than those of a driven piling, only in part because they do not have to withstand driven forces. Prominent among reasons for this is because they usually have a very much larger cross-section. It is not unusual for an in-situ piling to have a diameter as great as 36 inches, while a driven piling usually will be no larger than 18 inches in diameter, in large part because of the substantial skin friction that must be overcome to sink a piling. In-situ pilings do not face this problem. There is no skin friction to resist driving forces.
Also, because of their lower and affordable cost, there can be many more of them.
Compressive strengths as low as 40 psi are considered to be acceptable for many in-situ pilings, which may be as deep as 60 feet. Interestingly, these may be prepared in as short a time as 5 minutes. Thereafter they cure in times calculated in hours or days. Driven pilings are simply unable to compete with such a pace.
There are two basic generally-used methods to form in-situ pilings: the wet and the dry. The wet method injects a slurry of water, cement and/or lime into the bore as the auger either enters or leaves the bore, or at both times. The auger itself rotates vanes which both drill into the soil and mix the soil and injected slurry. The slurry is prepared in a mixing plant located on the surface. It is fed under pressure to the auger through pipes and hoses. The slurry is forced under pressure from the auger into the soil. It enters the soil as a strong stream. If the soil is dry, then a slurry injected and mixed into it would appear to be an ideal arrangement.
However, there are several serious disadvantages to this arrangement. The slurry in the lines, if permitted to stand too long such as during an interrupted operation for a substantial time, or overnight, will harden in the system. Then the system itself must be taken apart and cleaned out, or parts must be replaced as necessary. Also, unused slurry must be disposed of at the end of a work shift. This becomes an ecological problem. These are disadvantages at the surface and in the equipment. They do have the advantage that they can be “fixed”, but at a substantial cost.
The subsurface problems with the wet method are more severe. They are even more worrisome because they can and often do result in a deficient piling. Slurry to be pumpable and mixable in the soil must have some known amount of water of its own. If the amount of water in the slurry plus water in the formation is sufficient for hydration of the amount of cement and lime, then an in-situ piling formed with it in dry soil could be proper. The usual situation is that most soil (but far from all) has useful water in it, although not at all depths, and it can occur at different wetness at various depths.
Accordingly, a slurry of constant properties and composition can end up either not diluted or diluted to an unknown or excessive extent, unless it was precisely constituted for the immediate depth in the formation, which cannot effectively be done with mixing equipment at the surface which must be a continuous operation with long hose lines filled with already mixed slurry. In designing an in-situ piling using the wet method, the engineer must either accept a minimal load value or an over-design. Then he must over-pay for a larger piling, or for more pilings, or for extra binder, all of which can be prohibitively costly. These are serious disadvantages in days when money is short. Design criteria in excess of real requirements can not be tolerated, but is, in the absence of an alternative.
The dry method has even more severe restraints and consequences. In this method, dry cement and/or lime is mixed into the bore through the auger while the auger drives into the soil and stirs it. Existing water is relied on for the curing. Sometimes water is injected into the soil, but attention is rarely given to the variability of wetness at various depths. As a consequence, examinations of many completed in-situ pilings show various properties at different depths, extending from almost negligible strength near the surface where it is likelier to be drier, to excessive water potentially leading to reduced strength at depths where there was a deleterious excess of water when the piling was formed.
Applicant has developed a third method, with which he assures that at all pertinent depths there will be sufficient water to react with the binder he supplies, and also that there will be a proper amount of binder at each depth. The amounts of binder and of water supplied by this third method can and often will vary for depth to depth.
The objective is to produce at each depth a column having strength and dimensions suitable for each respective depth.
While so doing, energy loads on the equipment are significantly reduced. This is especially the situation when the soil is dry. Rotating and driving an auger in dry soil requires a substantial effort. The introduction of the water provides lubricity which reduces the energy load to drive the auger.
A further disadvantage of the prior art is the method of injecting the binder. It is customarily injected into the bore by a compressed air stream. The problem here is the distribution of the binder when it arrives in-situ. To obtain the best piling the binder should be evenly distributed, but pneumatic propulsion of a dry powder into a variable region often results in uneven distribution because of the nature of the formation into which it is injected. It may shoot all the way to the edge of the bore, or may be stopped quickly and never go very far into it. It then is the task of the auger to correct this by proper stirring of the entire mixture.
Applicant has found that the preparation of in-situ pilings with consistent and known properties depends heavily on the distribution of the water and binder in the bore, on the accurate and known presence and supply of each, and the nature of the soil in which the piling is formed. It must be kept in mind that while this process is relatively rapid, it still takes some time. For example, a 60 foot deep piling completed in five minutes requires axial auger movement at the rate of about 24 feet per minute. The usual rate of rotation of the auger is between about 150–250 rpm. Thus the auger travels axially at between about 15 and 30 mm per revolution. Accordingly, the binder is injected at a fairly rapid rate. However, its distribution and the water content of the soil at the point of injection is dependent on the nature of the soil—it is more difficult to penetrate clay than sand or sandy soil, for example, while in sandy soil water may drain quickly. Therefore, especially when fast-setting binders are used, there is the risk of earlier agglomeration of binder and water, and for slower-setting binders of a lesser amount of water because some water may have drained away. The injection of dry binder can vary also with the existing water content of the soil.
As to the addition of water, it is observed that the most troublesome situation occurs in sand or very sandy soil, from which existing and especially added water may drain away in important amounts before it is contacted by the binder. Thus, it is not only important to assure the presence of known amounts of water at various levels, but to have them there when the binder is added.
It is an object of this invention to provide process and equipment to enable local control over the injection of water and binder, and in such a way that the water and binder are in place in correct amounts at the time and place where they are to cure, and to mix them there. This invention provides the advantages of the wet method, but creating a slurry locally without the disadvantages of the wet method.